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JAWA modifications 1971 

Technical Report 1/71 September 1971

Modification 3825 Exhaust Silencers

As from 22 December 1970 (note advice is nearly one year later change was made)

The screw fastening the exhaust silencer tail pipe to the silencer on all motorcycles type 559, 592/00, 360/00 and 362/00/02 is a button headed screw which was replaced by a hexagon headed screw to ensure reliable tightening.

Old part no 6.1.1146.2.039 New number 6.1.1103.2.039 Screw M4 x8


Modification No 3940 from June 1970

Models 559/07, 592/00 and 360/00

The plastic edging (part no 559-39-013) on the fuel tank side cover has been omitted as tests show its superfluity. The fastening has also been modified. Fuel tanks with chrome plated sides discontinued. New screw M4 x6 part no 6.1.1155.2.038


Modification 3909 from 20 February 1971

All JAWA motorcycles - Coil fastening yoke modified part 353-66-011 replaced by part no 361-60-066.

Until exhaustion of original mounting yoke stock old yokes underlaid by felt or rubber pad 2mm thick.


Modifications 3992 and U506 from 14 April 1971

Carbon brush in dynamo upgraded. Part 150-19-024 replaced by 355-61-117 parts interchangeable.

New coil with resistor 603-61-150 replaces 355-61-117. New part already in use on D-623 dynamos.


Modification 3956 from October 1971

On JAWA Californian IV machines oil tank and oil level gauge will be modified. New tank will only take new gauge. Old gauge will remain as a spare part no 591-38-020




Oil tank



Oil level gauge complete



Sealing ring



Oil tank complete



Oil level gauge










Modification 3867 from September 1970

Engines 362-0-001565 frame 001525

Fuel tank mounting modified to prevent tank cracking from vibration. New mounting in silent blocks

Old tank 590-39-050 new tank 362-39-080

A new bracket part 353-31-311 was available and required an 8.4 mm hole drilling in the top frame tube.


Modification 3812 from 1st July 1971

Unifying parts for wheels after exhaustion of stocks. New parts fully interchangeable.




















Modifications 3633 and 3980 from May 1971 on Models 559,592,360 and 363

Glossy chrome plating on handlebars and wire spokes replaced by galvanised (glossy zinc plating) to render them more resistant to corrosion.


Modification 4037 from May 1971

Non return valve on oil pump superfluous in operation. Fitting and delivery of spare parts discontinued.


Modification 3831 and 3931 Californian IV

Cracking oil tank lugs remedy

3831 lug increased from 1.5 mm thick to 2 mm.

Thicker front lugs fitted since April 1970 and rear from 28 October 1970 to frames from 001726 and engine 002076 on.

3931 longer front lug and new method of welding. There should be no problems from 362-002351 onwards made since 12 February 1971. Repair shops asked to report defects on later serial numbered machines.


Modification 4025

To make production and sales documentation Californian IV 1970 conform with the actual facts substitute 361-43-030 with 361-43-020 and 361-43-035 with 361-43-025.

Note: Mototechna numbers remain unchanged as 316-43-030 nor 361-43-035 were never manufactured or delivered as a spare part.

Advice to Repair shops and Dealers

  1. Knocking of the big end bearings on JAWA 350 without signs of seizing or similar defect is usually caused by excessive axial clearance of the connecting rods between the flywheels. Strip crank - Solution fit a 2 mm steel washer. If clearance then exceeds 0.1 to 0.3 mm on either side fit a thicker washer.
  2. If the customers complain of reduced engine power (350 Californian IV Oilmaster and JAWA 250/350 types 623 and 633) make sure there is no rag or obstruction under the seat to the air intake. Make sure vent hole in oil tank cap is clear and remind owner to keep it clear (to prevent seizing due to no oil flow). Replace PAL plug 14-9 with 14-8 if carbon build up due to low temperature city traffic or running in.
  3. Oilmaster Californian IV increase power output. Models 362/00/00, 362/00/02 and 362/00/04 and 362/00/06. In 1970 an insert limiting the entrance of air was fitted inside the inlet pipe socket (stub nipple) in the intake silencer cover 591-01-110 to reduce noise. It was found that a carbon build up in the silencers has the same effect of reducing noise so the insert was unnecessary. Remove the insert and replace main jet 90 or 92 with jet 98. Results in increased power and acceleration. Inserts were dropped by the factory in the second quarter of 1971.
  4. Dealers must add 1 litre of oil to the oil tank after several tens of kilometres and before the motorcycle is driven to the customer to prevent seizing. Oil remaining in the tank after the factory functional test is insufficient and limited to avoid spillage in transit. All repairs will be charged to the dealer.
  5. When making motorcycles with oil pumps ready for winter drain the summer oil and refill with thinner oil winter oil. Avoid mixing oils and do NOT flush pump when changing oils.
  6. Californian IV catalogue change dynamo D-623 to D 623/3. The latter had a separate voltage regulator. Delete connecting rod complete part 633-12-145 and replace with 623-12-145
  7. On 15 November 1969 onwards PAL magnetos part 443-211-241.29 replaced the Lucas part on 500 DT Type 890 dirt track (speedway) motorcycles. Early PAL magnetos suffered stalling and weak spark caused by poor contact of the cable connection or lack of factory fitment of waterproof terminals. Remedy clean cable end and check/fit waterproof terminal. Check capacitor is tight and not displaced.
  8. To prolong spark plug life and sparking at the points later magnetos have a vented contact-point breaker space and capacitor locked against displacement. Older PAL magnetos should be fitted with a vent tube 120 mm long venting between the front engine plates. Ensure dust or water cannot enter or warranty claims will not be considered.


Here ends a fascinating insight into activity in 1970/71. I have used original phrasing when possible in reproducing this Technical Report. The original is now in the Club Library.

This is a further demonstration of the very small changes that occur during production. I guess the sequence of modification numbers was continuous so we can only wonder what the unpublished changes were. This article was prompted by Bob Clarks interest in changes to the CZ Twins. It is also my excuse if spare parts supplied to members are incorrect. Just return them and say why they are wrong.

I wonder if CZ produced similar technical reports?

By Pete Edwards

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